Partly silanized crosslinking agents based on hexamethylene diisocyanate are known from the prior art (EP 1 273 640 A2). The coatings obtained therewith feature scratch resistance and weathering stability, uniting as they do the advantages of purely organic coating systems with those of inorganic coatings.
These coatings are suitable in principle for all applications imposing heightened requirements in terms of mechanical loads. One specific case of an application where such mechanical properties are of interest is in automotive finishing. That application is not only marked by exacting requirements on the functional properties of the coatings, but likewise imposes exacting requirements on the decorative properties.
It is necessary, for instance, that the surface is particularly smooth and, where appropriate, that the basecoat situated beneath a clearcoat is supported much more than being impaired, in its capacity as a coloring coat. For this reason, particularly transparent and high-gloss clearcoat materials are desirable. At the same time, however, the other advantageous properties, such as high acid resistance, smoothness, and scratch resistance of the coating, are to be retained.
The gloss, as the value for the reflection of the light for a defined angle of incidence, depends heavily on the refractive index of the coat material. The greater the amount of light reflected, the greater the difference in the refractive index of ambient medium and coat. For this reason a brilliant, high-gloss surface is obtained when, for example, a polyester-based coat material is used, since polyesters possess a high refractive index. The refractive index of a coat material may likewise be increased through the addition of appropriate fillers such as silica, for example.
There has to date been no description of any process allowing the use of partly silanized, isocyanate-containing crosslinking agents to adjust the refractive index of the crosslinking agent and/or to increase the refractive index of the crosslinking agent.
A problem addressed by the present invention is therefore that of providing, for the production of a coating composition, crosslinking agents based on isocyanate compounds that exhibit a heightened refractive index, more particularly a refractive index nD20≧1.500.
A further problem addressed by the present invention was that of providing a coating composition (coat material) that leads to a coating (coat film) which exhibits increased light reflection and has a higher gloss than the coat films known from the prior art. The coating composition ought to be suitable more particularly for producing the clearcoat in OEM finishes and for use in automotive refinish systems.
Furthermore, the coat film ought also to meet the other functional requirements imposed on coatings and coat systems, more particularly on clearcoat systems within the technologically and esthetically particularly demanding field of automotive OEM finishing. The coating ought therefore to exhibit high acid resistance. In addition, the coating ought to have a high level of scratch resistance and ought more particularly to exhibit high loss retention after scratch exposure. Furthermore, the coating and coat system, more especially the clearcoat system, ought to be able to be produced even in coat thicknesses>40 μm without stress cracks occurring.
Furthermore, the application of the coating composition ought not to cause any environmental problems.